Typical examples of semiconductor devices are semiconductor devices in which a semiconductor chip (a silicon chip) is mounted on a frame, i.e., a substrate made of Cu or 42 alloy. Wire bonding using Au wires is carried out between a silicon chip and a frame. Bonding of a frame to a silicon chip sometimes uses an electrically conductive epoxy adhesive, but it is typical to use bonding by solder, which has high thermal conductivity and a large heat dissipating effect. This type of bonding is also referred to as internal bonding of semiconductor devices.
At the time of assembly of electronic equipment, semiconductor devices are mounted on printed circuit boards, and at this time they are subjected to heating by a second occurrence of reflow soldering. At this time, it is necessary that the soldered portions which previously underwent internal bonding not melt at the reflow temperature. Therefore, solder used for internal bonding of semiconductor devices employs solder alloys having a higher melting temperature than solder used for mounting on printed circuit boards. Such semiconductor devices themselves constitute semiconductor devices, but the semiconductor devices are further connected to circuit boards and constitute larger semiconductor devices. In this specification, solder which is used to carry out soldering of internal bonding of such semiconductor devices is referred to as high temperature solder.
In the past, high temperature solder was a Pb-based solder alloy having a melting temperature of around 300° C.
High temperature solders used for internal bonding of semiconductor devices include Pb-10Sn (solidus temperature of 268° C. and liquidus temperature of 302° C.), Pb-5Sn (solidus temperature of 307° C. and liquidus temperature of 313° C.), Pb-2Ag-8Sn (solidus temperature of 275° C. and liquidus temperature of 346° C.), and Pb-5Ag (solidus temperature of 304° C. and liquidus temperature of 365° C.) which primarily have Pb as a main component. These high temperature solders each have a solidus temperature of at least 260° C. Therefore, when using a 63Sn-37Pb eutectic solder, for example, for soldering for mounting on a printed circuit board, even if the soldering temperature at that time is a somewhat high level of 230° C., soldered portions for internal bonding of semiconductor devices which were soldered using a high temperature solder such as Pb-10Sn do not melt at the time of soldering for mounting on a printed circuit board.
However, from the standpoint of environmental protection, recently, over the entirety of soldering technology, there is a demand for the use of lead-free solder alloys in place of Pb-based solder alloys.
Naturally, there is also a demand to use lead-free solder alloys instead of Pb—Sn based high temperature solders like those described above which have been used for internal bonding of conventional semiconductor devices.
However, although there have been various proposals of lead-free solder alloys, there has been no high temperature solder alloy having Sn as a main component and having a solidus temperature of at least 260° C. For example, in the case of an Sn—Ag based solder alloy having a solidus temperature (eutectic point) of 221° C., increasing the Ag content increases the liquidus temperature but does not increase the solidus temperature. With an Sn—Sb based solder alloy having a solidus temperature of 227° C., when Sb is increased as much as possible in order to increase the solidus temperature, the liquidus temperature also greatly increases. The addition of other elements can also not change these properties.
Accordingly, from in the past, it has been thought impossible to use lead-free solder alloys as high temperature solders for internal bonding of semiconductor devices.
A bonding technique not using the high temperature solder alloys has been considered, i.e., a method of bonding using intermetallic compounds which have a high melting point compared to lead-free Sn-based solders.
Patent Document 1 discloses a method using a solder paste formed by mixing a powder of Sn or a powder of a lead-free solder having Sn as a main component with Cu powder. At the time of bonding by melting, bonding is carried out by forming an Sn—Cu intermetallic compound.
Patent Document 2 discloses one form of the invention described in Patent Document 1 as a solder material in which Sn powder or a lead-free solder powder having Sn as a main component and Cu powder undergo rolling to form a solder foil.